1. Field of the Invention
The present invention relates to a method for fabricating a color filter substrate for use in a color liquid crystal display (LCD) and other display devices.
2. Description of the Related Art
LCDs are relatively small, thin and lightweight display devices with comparatively low power dissipation. By taking advantage of these features, LCDs are currently used extensively in a broad variety of electronic appliances. Among other things, active-matrix-addressed LCDs with switching elements are used particularly widely in office automation (OA) appliances such as personal computers, audiovisual (AV) appliances such as TV sets, and cell phones. Meanwhile, the size, definition, effective pixel area ratio (i.e., aperture ratio), color purity and other quality parameters of LCDs have recently been increased or improved significantly.
The structure of a normal active-matrix-addressed LCD will be described with reference to FIG. 13, which is a cross-sectional view thereof.
As shown in FIG. 13, the LCD 30 includes an active-matrix substrate 2 and a color filter substrate 4, which are arranged so as to face each other, and a liquid crystal layer 6 provided between these two substrates 2 and 4. Also, as viewed perpendicularly to the principal surface of any of these substrates, the LCD 30 has an effective display area and a picture frame area that surrounds the effective display area.
The active-matrix substrate 2 includes a transparent insulating substrate 8 made of glass, for example, gate bus lines (not shown) to supply gate signals therethrough, source bus lines 10 to supply data signals therethrough, active components (not shown) such as thin-film transistors (TFTs) and transparent pixel electrodes 12. The gate bus lines, source bus lines 10, active components and pixel electrodes 12 are all provided on the substrate 8. The transparent pixel electrodes 12 are arranged in a matrix on the display area.
The color filter substrate 4 includes a transparent insulating substrate 14 made of glass, for example, a color filter layer 22 consisting of red color filters 16, green color filters 18 and blue color filters 20, an opaque layer 26 including a plurality of opaque portions 24, and a counter electrode (not shown). The color filter layer 22, opaque layer 26 and counter electrode are all provided on the substrate 14. The red, green and blue color filters 16, 18 and 20 are arranged so as to face their associated transparent pixel electrodes 12 on the active matrix substrate 2. The opaque layer 26 is arranged such that the opaque portions 24 are disposed in the gaps between the respective color filters and in the picture frame area.
Next, a conventional method for fabricating the color filter substrate 4 will be described.
Recently, a dry film process is often adopted as a method for fabricating such a color filter substrate. Hereinafter, a method for fabricating a color filter substrate by the dry film process will be described with reference to FIGS. 14A through 14F (see Japanese Laid-Open Publication No. 2001-100221, for example).
A dry film is a photosensitive resin layer, which is normally sandwiched between two film supporting members of polyethylene terephthalate (PET) films, for example. The photosensitive resin layer is one of four types of dry films, in which a red, green, blue or black pigment is dispersed, and is typically negative.
Specifically, first, a red dry film is attached onto, and rolled on, the glass substrate 14 and then its film supporting members are peeled off, thereby transferring a red photosensitive resin layer 16R onto the substrate 14 as shown in FIG. 14A. This process step is normally carried out with the dry film heated, i.e., a so-called “thermal transfer process”. Next, the red photosensitive resin layer 16R thus transferred is exposed to radiation through a mask 32 and then developed, thereby making red color filters 16 as shown in FIG. 14B.
Next, a similar process step is carried out on a green dry film to form green color filters 18 as shown in FIG. 14C. Furthermore, a similar process step is carried out on a blue dry film to form blue color filters 20 as shown in FIG. 14D. In this manner, a color filter layer 22, consisting of the red, green and blue color filters 16, 18 and 20, is obtained.
Thereafter, as in making the color filter layer 22, a black dry film is attached onto, and rolled on, the glass substrate 14, thereby transferring a black photosensitive resin layer 26R onto the substrate 14 as shown in FIG. 14E. Then, the black photosensitive resin layer 26R is exposed to a radiation that has come from under the back surface of the glass substrate 14 (i.e., a backside exposure process is carried out). As a result, the remaining portions of the black photosensitive resin layer 26R are masked and self-aligned with the existing red, green and blue color filters 16, 18 and 20, and then developed. In this manner, an opaque layer 26, of which the opaque portions 24 are arranged in the gaps between the adjacent color filters and in the picture frame area, is obtained.
The color filter substrate is obtained as described above.
Compared with a spin coating process that has been used quite often, the dry film process achieves a high material yield and can reduce the manufacturing cost significantly. Also, if the color filter layer and opaque layer are made of the dry films, then each of the two layers can have an even more uniform thickness.
However, in the manufacturing process described above, the opaque layer 26 is formed by subjecting a negative photosensitive resin layer to a backside exposure process as shown in FIG. 14E. Thus, the exposure dose needs to be controlled so as to prevent portions of the black photosensitive resin layer 26R on the color filters 16, 18 and 20 from being exposed to the radiation. For that reason, it is difficult to achieve a thickness or an optical density (OD) value which is sufficiently high for the remaining portions of the black photosensitive resin layer 26R to function as the opaque layer just as intended. Consequently, the resultant LCD often has a decreased contrast ratio. It should be noted that the OD value represents the transmittance of a substance. The higher the OD value of a substance, the lower the transmittance thereof.